Plasmatized heat exchanger

ABSTRACT

The invention relates to a temperature-controlled, rotating, rotational symmetric heat exchanger, particularly for the pressing mechanism, drying mechanism or smoothing mechanism of a machine for producing web-like products such as paper webs or plastic films. The heat exchanger can be temperature-controlled preferably with fluids or vaporous heat transfer media or with heating involving electrical means. The aim of the invention is to improve a heat exchanger of the aforementioned type as to achieve surface hardnesses of far greater than 400-450 HV and to minimize the chemical/mechanical corrosion even at heat transfer capacities &gt;80 kW/m. To this end, the heat exchanger is plamatized.

The invention pertains to a heat exchanger according to the introductory clause of claim 1. These types of heat exchangers are used in the production or finishing of web-like products in the industry, primarily in calenders.

The heat-transfer outputs and surface temperatures required in the new multi-nip calender rolls subject the conventional calender roll materials such as white cast iron, spheroidal graphite iron, and chromium alloy cast iron to loads which exceed their capacity. Forged steel is currently the material which is able to fulfill these requirements. Because adequate forged steel can be hardened and tempered only up to 400-450 HV without sacrificing the advantages of forged steel, the surfaces of these rolls are coated either by electrocoating or by a thermal coating process or they are case-hardened by inductive hardening or flame hardening.

Thermally applied surface coatings suffer not only from insufficient reliability but also from the disadvantage of the relatively high porosity of the finished layer. Wear particles are able to penetrate into these pores and into the softer matrix regions and to wash them out. Case-hardened forged steel rolls suffer not only from inadequate tempering properties and the associated lack of sufficient hardness but also from an unfavorable internal stress distribution, caused by the case-hardening process. High tensile stresses in the area of the zone affected by the hardening process destroy the advantages of the good mechanical properties of forged steel.

New coating compounds and feed materials being used in papermaking call for increased chemical and mechanical corrosion resistance not only of the barrel of the roll but also of the entire surface exposed to the environment and to the product web. Because of the enormous heat-transfer outputs (>80 kW/m) and the continually increasing heat-transfer temperatures, the contact surfaces exposed to the fluid heat transfer media are subject to ever greater chemical/mechanical corrosion.

The invention is based on the task of improving the heat exchanger of the general type in question in such a way that it no longer suffers from the disadvantages mentioned above, especially so that surface hardnesses of considerably more than 400-450 HV are reached, and so that chemical/mechanical corrosion is minimized even at heat-transfer outputs of >80 kW/m.

According to the invention, the previously described hardened and tempered forged steel rolls are plasma-treated in the desired areas. The term “plasma-treating” is used in the following as a collective term for “plasma nitriding”, “plasma nitrocarburizing”, “plasma oxidizing”, “plasma carbonitriding”, and “plasma carborizing”. This operation is usually carried out in a vacuum chamber at treatment pressures of 0.3-10 mbars. The treatment temperature is in the range of 400-600° C., depending on the task to be accomplished. Ammonia, nitrogen, methane, and hydrogen are usually used as the process gases. Pressure, temperature, time, and gas type are the parameters used to vary the nature of the layers and the desired surface hardnesses. Hardnesses of up to 1200 HV can be reached on the desired surfaces, and a hardening depth of up to 1 mm is realized. In addition to this extreme hardness, these diffusion layers do not produce any increase in the porosity which could be damaging to the wear process. The porosity of the substrate remains intact. In addition, the resistance to chemical corrosion is enormously improved.

Subsurface corrosion, known from thermal spray coatings or even hard chromium-alloy coatings, is not possible when plasma-treating is used. The bonding layer with the surface passivates the surface against oxidative attach. Microcracks, also known from the processes mentioned above, do not develop either. 

1. Temperable, rotatable, rotationally symmetric heat exchanger of hardened and tempered forged steel, especially for the pressing, drying, or smoothing unit of a machine for the production of web-like products such as paper webs or plastic films, which heat exchanger can be tempered preferably with fluid or gaseous heat-transfer media or by an electrical heating system, wherein the heat exchanger is plasma-treated.
 2. Heat exchanger according to claim 1, wherein the surfaces of the necks of the rolls are partially or completely plasma-treated.
 3. Heat exchanger according to claim 1, wherein the contact surface of the heat exchanger with the web-like product is plasma-treated.
 4. Heat exchanger according to claim 1, wherein the surface of the barrel of the heat exchanger is plasma-treated.
 5. Heat exchanger according to claim 1, wherein the surface of the roll is plasma-treated.
 6. Heat exchanger according to claim 1, wherein the surfaces in contact with the heat-transfer medium are completely or partially plasma-treated.
 7. Heat exchanger according to claim 1, wherein a bonding layer is formed at the surface.
 8. Heat exchanger according to claim 1, wherein no bonding layer is formed at the surface.
 9. Heat exchanger according to claim 1, wherein subsurface corrosion is prevented.
 10. Heat exchanger according to claim 1, wherein the plasma-treated calender roll can be used at process temperatures between −20° C. and 500° C. 